Method of coating luminescent tubes



Nov. 7, 1939. T. w. FRECH METHOD OF COATING LUMINESCENT TUBES Filed Aug. 24, 1958 Inve-VELCDT` Theodore W. Frech n 5%/ MM yI--iis ttorney.

Patented Nov. 7, 1939 UNITED" STATES PATENT OFFICE METHOD 0F COAT'ING LUIHINESCENT TUBES Theodore W. Frech, Cleveland Heights, h10, assignor to General Electric Company, a corporation of New York Application August 24,

` 2 Claims.

My invention relates to the manufacture of vitreous articles having a coating of luminescent material thereon, and more particularly to a method of applying such luminescent material to the inner walls of glass tubing.

Certain types of electric dischargedevices, such as fluorescent lamps, have a coating of iiuorescent material on the inner wall of the lamp envelope which material fluoresces or emits light when excited by the rays emitted by the discharge in the device. It is well recognized that, in order to produce a lamp of maximum efficiency, the coating of iiuorescent material must be of uniform thickness throughout. Various methods have heretofore been proposed for applying the fuorescent coating. For example, the most common way is to mix the fluorescent material with a suitable binder which is then sprayed over the surface to be coated. Needless to say, with ordinary spraying methods it is extremely diiiicult to secure a coating of uniform thickness. Other methods such as painting or the like have also been found incapable of producing a uniform coating.

One object of my invention is to provide an improved method for applying a uniform coating of luminescent material to the walls of glass articles.

Another object of my invention is to provide an improved method for applying a uniform coating of fluorescent material to the inner wall of a fluorescent lamp envelope.

Further objects and advantages of my invention will appear from the following description and from the accompanying drawing which is a front elevation, partly in section, of the apparatus I employ in the practice of my invention.

Referring to Fig. 1, the bulb or envelope Ill to be coated is placed inV the coating apparatus l shown in the drawing. This apparatus comprises lili till

a bottle or flask I I, preferably of spherical shape and formed with three parallel-extending neck or outlet portions, consisting of a central neck I2 and two diametrically opposite side neck portions i3 and I4. The esk II is rmly mounted on and secure to a suitable base member I5 by means of a cla ping ring I 6 and tie straps (not shown) extending from said clamping ring. As shown, the flask is mounted on the base I5 so that the neck portions I2, I3 and III extend upwardly. The said neck portions I2, I3 and I4 are provided with suitable rubber stoppers il, I3 and [I3 respectively. Extending through the stopper I1 into the body portion of the flesk I I is a propeller shaft provided at its inner end with a suitable propeller 2| or other type of agitator. The

1938, Serial No. 226,566

propeller shaft 20 is journalled in a combination bearing and stufllng box 22 extending through stopper I1 and secured thereto by a nut 23. The outer end of the propeller shaft 20 is coupled to the shaft 24 of a small electric motor 25 by means of a suitable coupling member 26. 'I'he motor 25 is secured by a mounting strap 21 to a support column or standard 2B extending upwardly from the base I5. The motor 25, ,together with the propeller shaft 20 coupled thereto, functions as an agitating device or mixer for a batch of coating material 25| with which the flask II is lled. v Y

A pressure-supply tube 30, preferably of glass, extendsv through the stopper I8 into the flask II. The said tube is connected by a hose 3| to a suitable source of compressed air or gas, preferably carbon dioxide. The tube 30 is also provided with a finger valve consisting of a short length of hose 32 the free end of which is open to the atmosphere. 'Ihe valve 32, when open, affords free escape of the compressed air or `gas entering the pressure-supply tube 30 through the hose 3I.

Mounted on the stopper I9 and extending therethrough down into the flask II is a feed tube 33. This feed tube extends to a point adjacent the bottom of the flask so that the open end of the tube will Vbe submerged in the reservoir of coating material 29 even though the said coating material, due to the using up of the same, recedes to a relatively low level. The upper portion of the stopper I3 is formed with an enlarged portion having a recessed seat 3E adapted to receive the open end of the tube or bulb I0 to be coated. The stoppers l1, I8 and i9 snugly fit in the corresponding flask neck portions I2, I3 and M so as to form a hermetic joint therebetween. Likewise, the joints or fits between the propeller shaft 2li and stopper ll, and between tubes 33, 33 and stoppers i3 and i3 respectively, are also made hermetic. Thus, when the pinch tube or valve 32 is closed, the compressed air or gas entering the ask Ill through tube 33 cannot escape and so forces the coating material 29, which is in a liquid state, up through the feed tube 33 into the tube or lamp bulb Ill to be coated. The elements which extend into the ask Il and which cone in contact with the coating material 23 are preferably made of stainless steel so as not to become rusted; otherwise contamination of the coating material 23 might result.

The coating material 29 I employ to produce the uorescent coating on the lamp bulb Ill consists of a mixture of binder and fluorescent material which is preferably made as follows: a binder is first prepared by dissolving 2350 grams of nitrocellulose solution in 14,900 cc. of amyl i acetate. The viscosity of the resulting mixture should be between 40 to 42 seconds as measured in a Dupont #7 cup at 28 C. To 900 cc. of the above binder is added 600 grams of fluorescent material previously ground to pass a 200 mesh silk bolting cloth. This mixture is`then stirred for approximately one hour in a ball mill so as to acquire a uniform consistency, whereupon 950 cc., approximately, of amyl acetate and 350 cc. of the previously mentioned binder is added and the mixture further stirred in a ball mill for approximately iifteen minutes. The mixture is then strained through a 150 mesh nickel screen and then an additional quantity of amyl acetate is adde togive the properconsistency, i. e., that consistencylfwhich will give the desired thickness of coating on'the lamp envelope I0 when applied by the method according to my invention.

To coat a tubular open-ended lamp bulb or tube I0 with uorescent material by the method comprising my invention, the tube is positioned vertically on the rubber stopper I0 with the open end of the tube rmly seated in the recessed seat 36 provided therefor. 'I'he valve 32 is then closed by pinching the same, whereupon the pressure of the CO2 gasentering the ask Il through the hose 3| and pressure-supply tube 30 forces the coating material 29 up through the feed tube 33 into the lamp bulb l0. When the level of the rising coating material reaches the top of the bulb I0, the pressure is released by opening the valve 32 so that the coating material ilows back into the fiasktl I. A deposit of coating material accordingly remains on the inner bulb walls. The coating material 29, during its application to the lamp bulb I 0, is continuously agitated by rotation of the-'propeller 2l of the agitating device.

Following the above initial application of'the coating material 29, the lamp bulb I0 is removed from the coating apparatus and placed vertically under a gentle stream of air, preferably at a pressure of approximately 14 to l5 lbs. per square inch, until the deposit of coating material is thoroughly dried. The vertical disposition of the tubular bulb allows Athe excess of coating material to drip olf. When dried, the lamp bulb is again placed in the coating apparatus and recoated, the bulb, however, in this second coating application, being inverted from the position occupied during the iirst coating application. By so inverting the bulb, an extremely uniform coatingis obtained. After this se'cond coating application. the lamp bulb is again placed in the drying apparatus and the coating material air dried as described above.

To burn oi the binder, the coated bulb l0, following the final drying, is iired while in a horizontal position for approximately 11 minutes inv an open-type oven at a temperature of approximately 460 C., the lamp bulb being moved through the oven in such a manner, and the gas oven burners being so adjusted as to gradually raise the temperature of the lamp bulb to the required firing temperature and then to gradually lower the bulb' temperature to that of the room. As a result, no cooling strains are created in the glass lamp bulb or 'tube I0.

A tube or envelope prepared in accordance with the above coating method has been found to possess extremely lmifonn thickness of uores- 4cent material, and it will be obvious that various changes and modifications may be made in the described procedure without departing from the spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of applying a 'coating of powdered luminescent material to the inner wall of a glass tube having both ends open which comprises mounting said tube in a vertical position, owing upwardly into said tube from the lower end thereof a coating solution containing a nely divided luminescent material in suspension, withdrawing saidl coating solution from said tube to thereby form a primary deposit of the solution on the inner wall of said tube, drying said primary deposit of coating solution while said tube is still in said vertical position, inverting said tube and again iiowing thereinto in the same manner as before the said coating solution, withdrawing said coating solution from said tube to thereby form a secondary deposit of the solution on the inner wall of said tube, and drying said secondary deposit of said coating solution while said tube is still in said inverted position.

2. 'Ihe method of applying a coating of powdered luminescent material to the inner wall of a glass tube having both ends open which comprises mounting said tube in a vertical position, flowing upwardly into said tube from the lower end thereof a coating solution containing a ilnely divided luminescent material in suspension in a volatilizable binder, withdrawing said coating solution from said tube to thereby form a primary deposit of the solution on the inner wall of said tube, drying said primary deposit of coatlng solution while said tube is still in said vertical position, inverting said tube and again flowing thereinto in the same manner as before the said coating solution, withdrawing said coating solution from said tube to thereby form a secondary deposit of the solution on the inner wall of said tube, drying said secondary deposit of said coatingsolution while said tube is still in said inverted position, and heating said tube suiciently to volatilize the binder in the coating solution.

THEODORE W. FRECH. 

